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From: jms@tardis.Tymnet.COM (Joe Smith)
Newsgroups: comp.sys.amiga
Subject: Re: Audio Improvements
Summary: 8, 12, or 16 bit into a 12-bit DAC
Message-ID: <538@tardis.Tymnet.COM>
Date: 25 Aug 89 06:08:56 GMT
References: <3114@bucsb.UUCP> <1542@convex.UUCP> <9051@attctc.Dallas.TX.US> <2450@pur-phy>
Reply-To: jms@tardis.Tymnet.COM (Joe Smith)
Organization: McDonnell Douglas Field Service Co, San Jose CA
Lines: 36

In article <2450@pur-phy> murphy@newton.physics.purdue.edu.UUCP (William J. Murphy) writes:
>I don't see the purpose in doing what you suggest.  If you have a 12-bit
>DAC, then with the 16-bit DMA we already have, why do you need to do
>the audio-HAM mode?  I would think that it would just be easier to do 
>12-bit audio directly.  Besides, the Audio-HAM mode would not reduce the
>storage requirements of a raw audio sample.

Right now we have an 8-bit DAC being driven by an 8-bit data stream.
Fibbonacci delta encoding allows driving an 8-bit DAC from a 4-bit stream.

With a 12-bit DAC, we could use a 16-bit data stream (wasting 4 bits),
a 12-bit stream (optimal), or an 8-bit stream.  If most of the changes in
the audio signal are less than +/- 63 units per time, using 8 bits per
sample wins over using 1.5 bytes per sample.

In article <9051@attctc.Dallas.TX.US> ltf@attctc.Dallas.TX.US (Lance Franklin) writes:
>Well, how about adding a "Ham-Mode" for the audio channel...
>Let's say, perhaps, that we have a 12 (or more) bit D-A on each channel.
>Each byte, when the channel is in this mode, is interpreted thusly:
>     Bit 7 : Mode bit...when 0, bits 6-0 will be loaded directly into the
>             high order 7 bits, with all low-order bits set to zero.
>             When set to 1, bits 6-0 are interpreted as a signed 7 bit
>             number which is added to the current value of the D-A 

byte 1 bit 7 off = put bits 6-0 into DAC bits 11-5 and zero DAC bits 4-0
byte 1 bit 7 on  = bits 6-0 are a number from -64 to +63.  Add this value
                   to the 12-bit number currently in the DAC.
byte 2 exactly the same as byte 1.

End result: 33% fewer bits required to store an audio sample (8 vs 12).
Zero distortion if the original audio signal is reasonably well behaved.
-- 
Joe Smith (408)922-6220 | SMTP: JMS@F74.TYMNET.COM or jms@tymix.tymnet.com
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